1. The Field of the Invention
The present invention relates to methods for the assembly of mechanical systems, in particular suspension devices intended for fitting on vehicles, and the scatter with respect to nominal orientation obtained during the manufacture thereof.
2. The Related Art
In an automotive vehicle, the wheels are held in position relative to the body by the suspension system. More precisely, the wheels are mounted on hubs which rotate, via a bearing or group of bearings, relative to a stub axle or wheel support connected to the suspension elements, which are themselves connected to the body. The precise orientation of the wheel axis (determined by the bearings) is an important element for the handling of the vehicle during use. For that reason automobile manufacturers strive for close tolerances as regards the final orientation of the wheel axis, such orientation being the result of assembling together numerous mechanical components connected to one another by articulations or rigid joints. A consequence of the principle of nominal value chains is that, to achieve a final nominal value (in this case a final angle of orientation) within a certain tolerance, the manufacturing tolerance of each element in the assembly must be a fraction of the final tolerance. Even disregarding the variability of the position of the attachment points to the body and considering only the suspension sub-assembly, it is still a real problem for manufacturers to obtain overall nominal values within the tolerances demanded.
Various solutions are known for this problem, which are implemented according to the type of systems concerned and the precision sought. For example, in the case of a xe2x80x9cmulti-armxe2x80x9d axle (so termed, because it consists of several arms which control the movements of the wheel support), the precision desired can be obtained by adjusting the system on a measurement bench after assembly. This adjustment is carried out by adapting the length of certain elements with the aid, for example, of a male and female screw joint or by the displacement of attachment points, for example by means of oblong holes. Solutions of this type necessitate a system designed to be adjustable (and therefore one that is heavier and/or more expensive), and a complex and costly adjustment step in the manufacturing process. Moreover, this adjustment is often lost during subsequent repairs of the vehicle, and the adjustment process must therefore be repeated. Another solution, one which is often used in the case of welded axles (such as rear torsion axles), is to machine certain reference surfaces after assembly of the elements, for example the plates on which the stub axles are fixed. This machining stage is also costly and necessitates the presence of a reserve of material designed to allow machining without compromising the strength of the system.
The object of the present invention is to reduce the statistical scatter of the final orientation of the wheel axis, while avoiding any final machining or introduction of supplementary adjustable elements.
The stub axle is generally fixed with bolts or by welding to a plate or in a bore. In general, in order to control the scatter of the final orientation of the stub axle, one checks the orientation of the plane of the plate, or that of the axis of the bore. This is for at least two reasons: on the one hand, the scatter introduced by the stub axle assembly is negligible because good precision can easily be obtained by turning, and on the other hand, the stub axle is not always part of the pre-assembled sub-group.
A principle of the invention is to use the stub axle mounting stage to reduce the final statistical scatter, by compensating at least in part the dimensional scatter of the other elements constituting the system. To do this, a stub axle is used whose axis forms a known correction angle with the axis of its surface designed to be supported on the plate (or in the bore), and the stub axle is fixed along an azimuth as a function of the angular deviation of the plate (or bore). In this way, the final scatter of the orientation of the axis of the stub axle can be statistically reduced.
Accordingly, the object of the invention is achieved by a process for the assembly of a stub axle on a suspension device intended for fitting to a vehicle, which suspension device comprises a surface that receives the stub axle. The stub axle comprises a stub axle axis and a contact surface with the suspension device, a characteristic axis of the contact surface forming a given correction angle xcex3c with the stub axle axis. The process comprises the following steps:
determining the azimuth xcex4d of an error in the orientation of an axis characteristic of the receiving surface; and
positioning the stub axle on the suspension device along an azimuth xcex4c essentially equal to xcex4d+xcfx80.
Preferred embodiments of the invention will be described to illustrate its principles, but the invention can be implemented in many other possible ways.